Hydrogels comprise water and polymers and are useful for medical and pharmaceutical applications (e.g., see Peppas, N. A.; Editor, Hydrogels in Medicine and Pharmacy, Vol. 3: Properties and Applications. 1987; p 195 pp.). Hydrogels are usually held together via physical or chemical crosslinks. Otherwise, the polymers of which they are comprised would dissolve in the solvent (e.g., water). Polyelectrolyte complexes are interpenetrating complexes of one or more predominantly positive polyelectrolytes and one or more predominantly negative polyelectrolytes. The opposite charges on the polymers form ion pairs between chains, holding the chains together. This ion pairing is a type of physical crosslinking. Polyelectrolyte complexes in contact with aqueous solutions can be considered hydrogels with high crosslinking density.
Polyelectrolyte complexes are prepared in a straightforward manner by mixing solutions of positive and negative polyelectrolytes. However, the resulting precipitate is gelatinous and difficult to process. The dried complexes, for example, are generally infusible and therefore cannot be injection molded or reformed into articles under elevated temperatures. Michaels (U.S. Pat. No. 3,324,068) has disclosed the use of non-volatile plasticizers such as nonvolatile acids, organic oxysulfur compounds, and organic oxyphosphorous compounds to decrease the brittleness of polyelectrolyte complexes when they are dried. U.S. Pat. No. 3,546,142 describes a method for creating solutions of polyelectrolyte complexes using aggressive ternary solvents which are mixtures of salt, water, and organic solvent. Said solutions of dissolved complexes may be cast into films by evaporating the solvent on horizontal plates. Mani et al. (U.S. Pat. No. 4,539,373) point out that the solid polyelectrolyte complexes “are not thermoplastic, i.e. they are not moldable or extrudable, so they must be handled as solutions.” Mani et al. disclose a polyelectrolyte complex comprising nonionic thermoplastic repeat units which can be thermally molded.
U.S. Pat. Nos. 8,114,918; 8,222,306; 8,283,030; 8,314,158; and 8,372,891 and U.S. Pat. Pub. No. 20090162640 which are incorporated fully by reference, disclose how fully hydrated (i.e., complexes in contact with water) polyelectrolyte complexes may be reformed into shapes without raising the temperature, without the addition of organic solvent, and without the need for dissolution, if they are doped with salt ions to a sufficient extent.
An alternative method for producing ultrathin films (less than about 1 micrometer thick) of polyelectrolyte complex is the multilayering method described by Decher in U.S. Pat. No. 5,208,111, wherein a surface is exposed in an alternating fashion to solutions of positive and negative polyelectrolytes. The resulting films are uniform and conformal, though ultrathin. The process, however, can be unacceptably slow, especially if numerous layers of polyelectrolyte are needed. As with any surface, roughness exists on the surface of these polyelectrolyte multilayer films, but the roughness is on the nanometer scale, typically in the range of 1-10 nm. Roughness exceeding the nm scale is considered undesirable, and may be minimized, for example by annealing the ultrathin film in salt solutions, as described in Langmuir, 17, 7725 (2001).
Regardless of the method of producing polyelectrolyte complex, for certain applications, there is a need to produce surface roughness in a finished product. Surface roughness of articles in contact with liquid can be advantageous in decreasing the viscous drag on said article. A well-known example is the surface of a shark skin, which is naturally rough. An additional benefit is that the turbulence at a micro-rough surface is thought also to decrease fouling or adhesion. The dual benefits of reduced drag and enhanced anti-fouling have motivated much research. U.S. Pat. No. 5,386,955 describes attempts to produce roughness in surfaces to mimic the shark skin. Roughness, or even a geometric pattern, can be embossed into a surface, as in U.S. Pat. No. 5,386,955, or the surface can simply be abraded. A coating method that spontaneously produces a rough surface on application would represent a significant advantage.